Several medical procedures require placement of a medical device in a lumen of human or animal body and in particular in the stomach. Once inserted, the movement of the inserted medical device has to be constrained and this is achieved by use of anchors. U.S. Pat. No. 9,636,245 describes a gastrointestinal device including a proximal element configured to reside in the stomach and a distal element configured to reside in an intestine, where the proximal element is configured to resist migration over time. US Pat. Application No. 2008/0058840 describes an alternative proximal element for placement within a hollow body organ. The proximal element includes a member having a first shape for delivery to the hollow body and a second shape for implantation therein. The member has sufficient rigidity in its second shape to exert an outward force against an interior of the hollow body so as to bring together two substantially opposing surfaces of the hollow body. The examples of implants described in U.S. Pat. No. 9,636,245 and US 2008/0058840 suffer from migration within a lumen of human body, and do not provide the long term required implantation stability.